Characterization of Plastic Flow Pertinent to the Evolution of Bulk Residual Stress in Powder-Metallurgy, Nickel-Base Superalloys

Abstract

The plastic-flow behavior which controls the formation of bulk residual stresses during final heat treatment of powder-metallurgy (PM), nickel-base superalloys was quantified using conventional (isothermal) stress-relaxation (SR) tests and a novel approach which simulates concurrent temperature and strain transients during cooling following solution treatment. The concurrent cooling/straining test involves characterization of the thermal compliance of the test sample. In turn, this information is used to program the ram-displacement-vs-time profile to impose a constant plastic strain rate during cooling. To demonstrate the efficacy of the new approach, SR tests (in both tension and compression) and concurrent cooling/tension-straining experiments were performed on two PM superalloys, LSHR and IN-100. The isothermal SR experiments were conducted at a series of temperatures between 1144 K and 1436 K (871°C and 1163°C) on samples that had been supersolvus solution treated and cooled slowly or rapidly to produce starting microstructures comprising coarse gamma grains and coarse or fine secondary gamma-prime precipitates, respectively. The concurrent cooling/straining tests comprised supersolvus solution treatment and various combinations of subsequent cooling rate and plastic strain rate. Comparison of flow-stress data from the SR and concurrent cooling/ straining tests showed some similarities and some differences which were explained in the context of the size of the gamma-prime precipitates and the evolution of dislocation substructure. The magnitude of the effect of concurrent deformation during cooling on gamma-prime precipitation was also quantified experimentally and theoretically.

The Effect of Cooling Rate on High-Temperature Precipitation in a Powder-Metallurgy, Gamma/Gamma-Prime Nickel-Base Superalloy

“…These variations would have led to small, non-uniform plastic strains which may have increased the effective diffusivity due to pipe-diffusion. In this regard, in an earlier investigation in which LSHR tensile bars were subjected to concurrent cooling and plastic straining at constant rates, [51] secondarygamma-prime precipitates whose sizes were 60 pct greater than those found in bars that had been cooled at the same rates but without concurrent straining were developed. Using the same fast-acting simulation method as here and in Reference 33 these larger sizes were rationalized on the basis of an effective diffusivity which was 2.5 times higher than that applicable to a strain free material.…”

Development and industrial application of integrated computational materials engineering

“…These efforts have provided accurate and appropriate properties, but these types of physical tests and measurements are not readily accessible to industry, but they can be used to validate first-principles models that can be further used by industry to predict difficult to measure liquid properties (Woodward 2008, Woodward 2012. Collaborative efforts to establish manufacturing relevant materials data by both the development of advanced physical testing methods and data analytics have supported industrial implementation of ICME tools and methods (Semiatin 2015, Zhang et al 2021. Materials property data required for manufacturing process simulation is a technology area of opportunity for further improvement through the potential development of industry standard measurement and analysis methods, and through validated first principles calculations.…”

On the Rapid Assessment of Mechanical Behavior of a Prototype Nickel-Based Superalloy using Small-Scale Testing

“…[24] Several recent studies have made use of SRT for the mechanical characterization of superalloys and have shown results consistent with conventional creep data from the literature, thus enabling accelerated testing campaigns of new alloys. [52][53][54] Here, in common with such approaches, it is assumed that the total strain rate of the system remains in equilibrium as crosshead displacement is fixed, consistent with:…”